The present invention relates to a method for preparing cyclotetrasiloxanes from chlorosilanes without the concomitant preparation of hydrogen chloride or hydrochloric acid. More specifically the present invention relates to a method for preparing cyclotetrasiloxanes from chlorosilanes and an acyloxy compound and coproducing an aliphatic chloride and/or an acyl chloride.
Cyclotetrasiloxanes, particularly cyclotetradiorganosiloxanes such as cyclotetradimethylsiloxanes, are valuable materials because they can be further polymerized to prepare higher molecular weight silicone compositions, such as oils, gums and resins, and because they are useful, without further processing, as fluids.
It is well known that dihydrocarbyldichlorosilanes can be hydrolyzed to prepare poly(dihydrocarbylsiloxanes) and hydrochloric acid. This method, although finding extensive commercial use, has several disadvantages. For example, the by-produced hydrochloric acid is an undesired by-product because it is corrosive, it cannot be used directly to form more dihydrocarbyldichlorosilane and it frequently reacts further with the hydrocarbyl radicals, such as methyl and vinyl radicals. Furthermore, this hydrolysis process does not produce only cyclopolysiloxanes, much less only cyclotetrasiloxanes, but rather a mixture of various linear and cyclic polysiloxanes. Also, this hydrolysis process cannot be used to prepare cyclopolysiloxanes bearing water-sensitive radicals such as silicon-bonded chlorine atoms or acyl chloride radicals.
U.S. Pat. No. 4,108,882 (Mahone) addresses some of the above-noted disadvantages and relates to a process for reacting certain alkyl silanes bearing 1 or 2 silicon-bonded chlorine atoms with methanol in the presence of a quaternary ammonium halide salt catalyst to provide certain alkylpolysiloxanes, including cyclopolysiloxanes, and methyl chloride. As noted in Mahone's patent, this process produces methyl chloride which can be used directly to produce more methylchlorosilanes. However, Mahone's process also produces hydrogen halide, which is undesirable, and, furthermore, the process does not provide cyclopolysiloxanes which bear water-sensitive radicals such as silicon-bonded chlorine atoms or silicon-bonded acyl chloride radicals.
U.S. Pat. No. 3,803,195 (Nitzsche et al.) also addresses some of the above-noted disadvantages and relates to a method for preparing organopolysiloxanes and alkyl halides which comprises passing organohalosilanes and organic compounds of the formula ROR' countercurrently through a reaction zone heated to 20.degree. to 150.degree. C. and containing an essentially inert, acid-resistant packing material of selected surface area. As noted by Nitzsche et al. this method produces alkyl halides which can be used directly to form more alkylhalosilanes. However, Nitzsche et al. discloses that aqueous hydrogen halides are formed as a by-product, in minor amounts, and the packing material must consequently be acid-resistant.
The organic compounds of the formula ROR' that are used in the Nitzsche et al. method encompass alkanols, such as methanol; dialkyl ethers, such as dimethyl ether; alkanoic acid alkyl esters, such as methyl acetate; and mixtures thereof, such as a mixture of methanol and methyl acetate. However, Nitzsche et al. does not teach how to use alkanoic acid alkyl esters in the disclosed process.
Nitzsche et al. further teaches that, although the ratio of organic compound ROR' to organohalosilane is not critical, there should preferably be from 1.0 to 1.75 moles of the ROR' compound for every gram atom of halogen in the organohalosilane when the ROR' compound is an alkanoic acid alkyl ester. Surprisingly, I have found that when said ratio has a value substantially less than 1.0, specifically from 0.25 to 0.5, depending upon the halosilane, there is produced an optimum yield of cyclotetrasiloxane and aliphatic chloride and/or acyl chloride.
Nitzsche et al. teaches that the method disclosed therein produces cyclic and/or linear organopolysiloxanes, however, if exclusively cyclic organopolysiloxanes are desired the coproduced linear organopolysiloxanes must be separated from the cyclic product and recycled through the reaction vessel being used in the method of the invention. Surprisingly, I have found that the method of this invention produces cyclotetrasiloxanes as substantially the only siloxane reaction product.
There is thus provided by this invention a method for producing cyclotetrasiloxanes from chlorosilanes that has many advantages over the processes of the art and avoids many of the disadvantages of the processes of the art.